A standard drill has a drive unit having a front end and an output shaft projecting from the front end and centered on and rotatable about an axis. A chuck body fixed on the shaft is formed with a rear end juxtaposed with the drive-unit front end, an axially forwardly open tool recess on the axis, and a plurality of angularly spaced and angled guides opening into the recess. Respective jaws displaceable in the guides have front ends in the recess. An adjustment sleeve rotatable about the axis on the body engages via formations such as screwthreads with the jaws for displacing the jaws radially on rotation of the sleeve about the axis. The shaft can also axially reciprocate or be provided with a reciprocating core shaft for hammer-drill action.
It is known, for example from commonly owned U.S. Pat. Nos. 4,607,855 and 4,627,628, to form the guides as outwardly opening holes so that any chips, oil drops, or the like that accumulate in the chuck can be thrown radially off, reducing fouling of the chuck mechanism. Nonetheless fine particles can build up and form a mechanism-clogging crust in the chuck. As a result the parts become unable to move freely relative to each other for self-tightening action or the like as described in commonly owned U.S. Pat. No. 5,286,041. The clamping force of the chuck is therefore reduced.